Optical data transmission may be coded in different formats. Two commonly used formats are either a return-to-zero signaling scheme or a non-return-to-zero signaling scheme. Return-to-zero (RZ) signaling refers to a signaling scheme in which the signal returns to zero between each pulse. The signal returns-to-zero between pulses even if a number of consecutive zeros or ones occur in the signal. Since the signal returns to zero between each pulse, a separate clock signal is, typically, not needed in the RZ signaling scheme.
Non-return-to-zero (NRZ) refers to a signaling scheme in which logic highs are represented by one significant condition and logic lows are represented by another significant condition with no neutral or rest condition. Since the pulses do not have a rest state, a synchronization signal is typically sent alongside the data signal.
For optical transmission, the data with return-to-zero (RZ) format provides more margins to the required optical signal-to-noise ratio (OSNR) and polarization-mode-dispersion (PMD) tolerance. The duty cycle of the RZ pulse has a direct impact on the transmission performance. At present, the RZ pulse shaping or duty cycle adjustment is realized in the optical domain by use of an additional Mach-Zahnder modulator (MZM).
MZM RZ conversion involves generating the RZ data pattern by implementing two optical modulators in optical domain. The first optical modulator is driven at twice of the data rate by a sinusoidal signal, and the resulting optical output signal is RZ optical pulse trains at 50% duty cycle.
The output RZ pulse train is then input to the second optical modulator which is driven by an NRZ data encoded electrical signal. The resulting output is optical RZ data pattern at 50% duty cycle. The second optical modulator is often called a carver since it actually encodes the data. To generate RZ signals at 33% or 67% duty cycles, the first optical modulator has to be driven at half of the data frequency by utilizing the characteristics of optical MZM.
In this conventional approach, two optical modulators along with two drivers are used, which is very expensive considering the cost of the second MZM and its driving electronics. The addition insertion loss from the second MZM and the lager form factors are the other disadvantages of the optical RZ encoders.